The present invention relates to atomic frequency standards and more particularly concerns an optical-pumping atomic frequency standard having a low control time constant.
Atomic frequency standards of the optical pumping type are well known. As shown in FIG. 1, they essentially comprise an optical pumping device 10 incorporating a light detector 24, a quartz oscillator 12 and an electronic system 14 for deriving from the oscillator a high frequency wave which is applied to the device 10 and for comparing the resonance frequency of the atomic system to that of the oscillator for controlling the oscillator.
In the device 10, optical pumping causes a reversal in population between the hyperfine levels of the ground state of atoms which are generally those of an alkali metal such as potassium, sodium or rubidium. In the case of a rubidium (Rb)-type frequency standard, one conventional optical pumping assembly is constructed as follows. A cell 16 which is subjected to a constant magnetic field contains the Rb 87 isotope, the spectrum of which comprises the two hyperfine components A and B. The cell is illuminated by a rubidium 87 lamp 18 through a filter 20 which contains the isotope Rb 85, the absorption spectrum of which comprises the hyperfine components a and b. The components A and a are virtually coincident, while the components B and b are completely separated. The component A of the emission spectrum of the lamp 18 is thus eliminated by the filter 20 so that the light which reaches the cell 16 is formed for the major part by the line B. Only the atoms of Rb 87 of the cell 16, which are in the lower hyperfine level (F=1), absorb light and are raised into higher states. They return by spontaneous emission either to the higher hyperfine level (F=2) or to the lower hyperfine level. As the atoms are immediately excited by the arrival of the light, the lower level (F=1) empties, to the benefit of the higher level (F=2). There is therefore a reversal in respect of the population as between those two levels and accordingly the cell 16 becomes virtually transparent to the radiation from the lamp 18. The cell 16 is disposed in a microwave cavity 22 which is excited at a frequency close to 6835 MHz, corresponding to the energy gap between the hyperfine levels F=1, m.sub.F =o and F=2, m.sub.F =o, which causes the hyperfine transition, accompanied by a stimulated emission of electromagnetic radiation between those two levels. As soon as the atoms which participate in the stimulated emission arrive at the lower hyperfine level (F=1), they are optically pumped and raised into the excited states. The greater the number of stimulated transitions, the greater the amount of light absorbed in the cell, while the smaller the amount of light arriving at a photoelectric cell 24, the lower is the current in the photoelectric cell 24. The photoelectric current passes through a minimum when the frequency of the cavity excitation signal is exactly equal to the transition frequency.
The walls of the cell are advantageously covered with a layer which is referred to as a "non-disorienting" layer and which is intended to eliminate the spin disorientation of the atoms when they come into contact with the walls. The non-disorienting layer makes it possible substantially to increase the duration of interaction of the atoms of the cell with the microwave field and thus makes it possible to have a narrow line of absorption of light in the cell 16.
The quartz oscillator 12 produces a 5 MHz signal which is modulated in a phase modulator 26 at a relatively low frequency (typically of the order of 100 Hz) produced by a low frequency oscillator 28. The modulated signal is applied to a multiplier 30 to produce a signal at the frequency of the stimulated emission, of 6835 MHz. It is this signal which is used to excite the microwave cavity 22.
The signal produced by the photoelectric cell 24 is amplified at amplifier 32 and then applied to a synchronous detector of phase comparator 34 which also receives a signal from the generator 28 so as to effect synchronous detection, making it possible to detect whether the carrier frequency of the signal applied to the cavity 22 is properly centered on the frequency of the hyperfine transition (6835 MHz). Any displacement results in an error signal at the output of the phase comparator 34. This signal is applied to an integrator 36 and then used to control a variable capacitor 38 which is coupled to the oscillator 12 and which modifies the frequency thereof so as to maintain the harmonic of the quartz oscillator centered on the frequency of the hyperfine transition of rubidium.
Frequency standards of the above-described type are systematically designed to achieve the maximum possible levels of performance in regard to stability. The width of the absorption line is therefore relatively small (of the order of 300 Hz), which makes it necessary to use relatively low modulation frequencies (generally less than 200 Hz). The immediate consequence of this low-frequency modulation is that the control time constant is relatively high (generally more than 100 milliseconds).